Description of the Related Art
Abnormal accumulation of extracellular amyloid in plaques and cerebrovascular deposits are characteristic in the brains of individuals suffering from Alzheimer's disease (AD) and Down's Syndrome (Glenner et al., BBRC, 120:885-890, 1984; Glenner et al., BBRC, 120:1131-1153, 1984). The amyloid deposited in these lesions, referred to as .beta.-amyloid peptide (BAP), is a poorly soluble, self-aggregating, 39-43 amino acid (aa) protein which is derived via proteolytic cleavage from a larger amyloid precursor protein (APP) (Glenner et al., ibid.; Kang et al., Nature 325:733-736, 1987). BAP also is thought to be neurotoxic (Yankner et al., Science 245:417-420, 1990). APP is expressed as an integral transmembrane protein (Dyrks et al., Embo. J., 7:949-957, 1989) and is normally proteolytically cleaved by "secretase" (Sisodia et al., Science, 248:492-495, 1990; Esch et al., Science, 248:1122-1124) between BAP-16K (lystine) and -17L (leucine). Cleavage at this site therefore precludes amyloidogenesis (Palmert et al., BBRC, 156:432-437, 1988) and results in release of the amino-terminal APP fragment which is secreted into tissue culture medium (Sisodia et al., ibid., Esch et al., ibid.). Three major isoforms of APP (APP-695, APP-751 and APP-770) are derived by alternative splicing (Ponte et al., Nature 331.:525-527, 1988; Kitaguchi et al., Nature 331:530-532, 1988; and Tanzi et al., Nature 331:528-530, 1988) and are expressed as integral transmembrane proteins (Kang et al., Nature 325:733-736, 1987; Dyrks et al., EMBO J. 7:949-957, 1988).
Even though both APP-770 and -751 isoforms contain a protease inhibitor domain, it is the secreted portion of APP-751 (also known as Protease Nexin II (Van Nostrand et al., Science, 248:745-748, 1990) which is thought to be involved in cell adhesion (Schubert et al., Neuron, 3:689-694, 1989), remodeling during development, coagulation (Smith et al., Science, 248:1126-1128, 1990) and wound repair.
Disease related mutations in the APP gene are found either within BAP sequences or near the BAP domain. A mutation within BAP (BAP.sub.E22Q) is found in APP of patients with hereditary cerebral hemorrhage with amyloidosis of Dutch origin (HCHWA-D), a condition in which a cerebrovascular BAP deposition is associated with stroke, and may be due to alteration in the rate of BAP aggregation (Wisniewki et al., Biochem. Biophys. Res. Commun. 179:1247-1254, 1991). A KM to NL double substitution two residues immediately N-terminal to BAP, which occurs in APP of patients with a particular form of early onset familial Alzheimer's disease (FAD), has been linked to the overproduction of BAP in tissue culture models (Citron et al., Nature 260:672-674, 1992). In another form of FAD, several mutations have been identified within the transmembrane-spanning domain of APP C-terminal to BAP at codon 717 (APP-770; V to F; I or G) (Kosik, Science 256:780-783, 1992). It has been suggested that these mutations alter normal coupling of APP to G-proteins (Nishimoto et al., Nature 362:75-79, 1993).
Although the mechanisms underlying proteolytic processing of APP are poorly understood, BAP is currently regarded to be central to the pathogenesis (Selkoe, Neuron, 6:487-498, 1991; Isiura, J. Neurochem. 56:363-369, 1991) and memory loss (Flood et al., Proc. Natl. Acad. Sci. 38:3363-3366, 1991) associated with Alzheimer's disease. It has been reported in the literature that BAP may be neurotoxic (Kowall et al., Proc. Natl. Acad. Sci. U.S.A. 88:7247-7251, 1991; Pike et al., Eur. J. Pharmacol. 207:367-368, 1991). Synthetic BAP (Yankner et al., Science 250:279-282, 1990) or purified plaques from Alzheimer's disease patients (Yankner et al., Science 245:417-420, 1989) are toxic to hippocampal cells in culture and neurons in rat brain, respectively. Recent reports suggest that BAP is involved in activation of the complement cascade leading to inflammation with potential neurotoxic consequences (Rogers et al., Proc. Natl. Acad. Sci. U.S.A. 89:10016-10020, 1992).
It has been observed that (a) amyloid plaques develop in AD brains, (b) a major component of plaques is BAP, (c) BAP is generated by proteolytic cleavage of APP protein, (d) mRNA levels of specific APP isoforms increase in AD suggesting that more APP protein is expressed, (e) APP point mutations which are thought to possibly alter normal processing have been identified in Familial AD (FAD) and "Dutch" disease, (f) injection of BAP into the brains of rodents both form lesions reminiscent of plaque pathology and result in memory deficits, and (g) plaque-like amyloid deposits have been detected in the brains of transgenic mice expressing human APP.